It is often necessary to attach objects to bone. For example, in repairing a fractured or damaged bone in a human the segments or fragments of bone must be fixated together to promote proper healing. Fixation may be accomplished by attaching a plate to the exterior of all of the bone segments.
Bone is made up of a hard exterior layer, known as the cortex, and soft interior tissue, known as cancellous bone or marrow. The cortex encompasses the entire outer surface of the bone. The cortex serves as a rigid, stable structure for attaching objects.
Prior art techniques utilize screws to secure a plate to a bone. To securely anchor a screw in the bone structure, bicortical placement of the screw in the bone is required. That is, the screw penetrates through the cortex layer that is adjacent to the object which is to be attached, then penetrates through the cancellous tissue in the interior of the bone and finally, penetrates into the opposite cortex layer on the other side of the bone. If the screw is not anchored into the opposite cortex layer, then the screw will wobble and will fail to securely couple the object to the bone.
Using screws in some situations increases the risk of the surgical procedure. For example, when fusing vertebrae together with a plate, the plate is coupled to the individual vertebra with screws. Each screw must extend through the anterior cortex, the cancellous tissue and into the posterior cortex. The tip of the screw must extend sufficiently far enough into the posterior cortex so as to be firmly anchored. However, the tip of the screw should not penetrate all the way through the thin posterior cortex because the spinal cord lies behind the posterior cortex. Any contact between the screw and the spinal cord would result in severe injury to the patient. The prior art relies on a ligament that is interposed between the posterior cortex and the spinal cord to provide some measure of safety by providing a space between the posterior cortex and the spinal cord. In addition, the prior art relies on the use of high resolution fluoroscopy to image the screw, wherein the surgeon can have an idea of where the tip of the screw is located. However, any improvement to this prior art procedure of attaching plates to vertebrae would no doubt be welcomed.
Another prior art technique of attaching objects to bone involves the use of toggle bolts. For example, in Fischer, U.S. Pat. No. 3,896,504, toggle bolts are used to secure a hip socket to bone. The installation of toggle bolts takes time and applies torque to the bone. In addition, once a toggle bolt is installed, removal is very difficult and time consuming. Removal is necessary, for example, if the toggle bolt is incorrectly installed.